Electrochemical process to recycle aqueous alkali chemicals using ceramic ion conducting solid membranes

ABSTRACT

A method for producing an alkali metal hydroxide, comprises providing an electrolytic cell that includes at least one membrane having ceramic material configured to selectively transport alkali metal ions. The method includes introducing a first solution comprising an alkali metal hydroxide solution into a catholyte compartment such that said first solution is in communication with the membrane and a cathode. A second solution comprising at least one alkali metal salt and one or more monovalent, divalent, or multivalent metal salts is introduced into an anolyte compartment such that said second solution is in communication with the membrane and an anode. The method includes applying an electric potential to the electrolytic cell such that alkali metal ions pass through the membrane and are available to undertake a chemical reaction with hydroxyl ions in the catholyte compartment to form alkali metal hydroxide.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of U.S. patentapplication Ser. No. 12/062,458, filed Apr. 3, 2008 which claimedpriority to and the benefit of U.S. Provisional Patent Application No.60/909,735, filed Apr. 3, 2007, which applications are incorporated byreference.

GOVERNMENT RIGHTS

This invention was made in part with government support under grantnumber DE-FG07-04ID14622 awarded by the United States Department ofEnergy. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

The present invention relates in general to a two-compartmentelectrolytic cell using an alkali cation-conductive ceramic membrane andto electrochemical processes performed in a two-compartment electrolyticcell using alkali cation-conductive ceramic membranes. Morespecifically, the invention relates to a two-compartment electrolyticcell, and a multi-compartment configuration of the electrolytic cell,and to a process to recycle and synthesize value added alkali chemicalsfrom an aqueous or non-aqueous waste stream containing alkali cationsand mixed monovalent and/or multivalent cations.

Many industrial processes produce aqueous and non-aqueous waste streamscontaining alkali metal salts in combination with other cations. Someprocesses, such as nuclear power plants, produce waste solutions thatcontain various cations, and in some cases radionuclides. Such wastesolutions may include, but not limited to, Na, K, Cs, Ca, Sr, Ba, Al,etc. It would be an advancement in the art to selectively recover alkalimetals (Li, Na, K) from the waste solution containing mixed cationswhile producing a useful alkali product.

BRIEF SUMMARY OF THE INVENTION

In accordance, there is provided a method of recycling and making alkalichemicals, acids and hydroxides, preferably from complex sodium oralkali salts, and a combination of alkali salt solutions. The methodcomprises feeding a salt solution, preferably into a catholytecompartment of an electrolytic cell, feeding an alkali metal saltsolution or combination of salt solutions, into an anolyte compartmentof the cell, and applying potential across the electrodes of the cell.The anolyte compartment and the catholyte compartment of the cell areseparated by an alkali ion conductive ceramic membrane that, uponapplication of the electric current, selectively transports the specificalkali metal cations from the anolyte compartment to the catholytecompartment. The membrane is substantially impermeable to water,operates at a high current density, and/or operates at a low voltage.The alkali metal cations, following their transport across the membrane,react with the corresponding anions in the catholyte compartment of thecell.

In this process, salts are decomposed in an electrochemical cell andselected alkali ions are transferred across an alkali ion conductingsolid electrolyte configured to selectively transport alkali ions.Oxidation and reduction are the principal electrolysis reactions at theelectrodes and depending on the nature of the salts other gas speciesevolve at the electrodes as product gases. The reduction of water at thecathode generates hydroxyl ions and hydrogen. As the sodium ions migratethrough the membrane from the anolyte side of the cell to the catholyteside, they will combine with the hydroxyl ions produced by the reductionof water to form sodium hydroxide solution.

The separator is preferably an alkali ion conducting solid electrolyteconfigured to selectively transport alkali ions. It may be a specificalkali ion conductor. For example, the alkali ion conducting solidelectrolyte may be a solid MeSICON (Metal Super Ion CONducting)material, where Me is Na, K, or Li. The alkali ion conducting solidelectrolyte may comprise a material having the formulaMe_(1+x)Zr₂Si_(x)P_(3−x)O₁₂ where 0≦x≦3, where Me is Na, K, or Li. Otheralkali ion conducting solid electrolytes may comprise a material havingthe formula Me₅RESi₄O₁₂ where Me is Na, K, or Li, where RE is Y, Nd, Dy,or Sm, or any mixture thereof. The alkali ion conducting solidelectrolyte may comprise a non-stoichiometric alkali-deficient materialhaving the formula (Me₅RESi₄O₁₂)_(1−δ)(RE₂O₃.2SiO₂)_(δ), where Me is Na,K, or Li, where RE is Nd, Dy, or Sm, or any mixture thereof and where δis the measure of deviation from stoichiometry. The alkali ionconducting separator may be beta-alumina. In specific embodimentsdisclosed herein, the alkali ion conducting solid electrolyte is aNaSICON (Sodium Super Ionic CONductors) cation ceramic membrane.

Such processes and devices for conducting such processes are disclosedherein.

Other advantages and aspects of the present invention will becomeapparent upon reading the following description of the drawings anddetailed description of the invention. These and other features andadvantages of the present invention will become more fully apparent fromthe following figures, description, and appended claims, or may belearned by the practice of the invention as set forth hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In order that the manner in which the above-recited and other featuresand advantages of the invention are obtained will be readily understood,a more particular description of the invention briefly described abovewill be rendered by reference to specific embodiments thereof that areillustrated in the appended drawings. Understanding that these drawingsdepict only typical embodiments of the invention and are not thereforeto be considered to be limiting of its scope, the invention will bedescribed and explained with additional specificity and detail throughthe use of the accompanying drawings in which:

FIG. 1 is a schematic view of a two-compartment electrolytic cellcomprising an alkali-cation conductive ceramic membrane according to thepresent invention.

FIG. 2 shows a graph of cell voltage as a function of time for Example1.

FIG. 3 shows a graph of cell voltage as a function of time for Example2.

FIG. 4 shows a graph of cell voltage as a function of time for Example3.

FIG. 5 shows a graph of cell voltage and current density as a functionof time for Example 4.

FIG. 6 shows a graph of cell voltage and current density as a functionof time for Example 5.

FIG. 7 shows a graph of cell voltage and current density as a functionof time for Example 6.

FIG. 8 shows a graph of cell voltage and current density as a functionof time for Example 7.

FIG. 9 shows a graph of cell voltage and current density as a functionof time for Example 8.

FIG. 10 shows a schematic view of a multi-compartment electrolytic cellcomprising alkali-cation conductive ceramic membranes and a bipolarelectrodes.

FIG. 11 shows a schematic view of a multi-compartment electrolytic cellarranged in series comprising alkali-cation conductive ceramic membranesand bipolar electrodes.

FIG. 12 shows a schematic representation of the multi-compartmentelectrolytic cell of FIG. 11 in which the alkali hydroxide produced inone catholyte compartment is introduced into a second catholytecompartment to increase the concentration of the alkali hydroxideproduced within the second catholyte compartment.

FIG. 13 shows a schematic representation of the multi-compartmentelectrolytic cell of FIG. 11 in which the alkali hydroxide produced fromseveral catholyte compartments are collected into a single catholyteoutlet stream.

FIG. 14 shows a schematic view of a multi-compartment electrolytic cellarranged in parallel comprising alkali-cation conductive ceramicmembranes.

DETAILED DESCRIPTION OF THE INVENTION

The presently preferred embodiments of the present invention will bebest understood by reference to the drawings, wherein like parts aredesignated by like numerals throughout. It will be readily understoodthat the components of the present invention, as generally described andillustrated in the figures herein, could be arranged and designed in awide variety of different configurations. Thus, the following moredetailed description of the embodiments of the electrolytic cell usingalkali cation-conductive solid ceramic membranes of the presentinvention, and processes using the two-compartment and multi-compartmentelectrolytic cell as represented in FIGS. 1 through 14, is not intendedto limit the scope of the invention, as claimed, but is merelyrepresentative of presently preferred embodiments of the invention.

The phrase “substantially impermeable to water,” when used in theinstant application to refer to a membrane, means that a small amount ofwater may pass through the membrane, but that the amount that passesthrough is not of a quantity to diminish the usefulness of the presentinvention. The phrase “essentially impermeable to water,” as used hereinin reference to a membrane, means that no water passes through themembrane, or that if water passes through the membrane, its passage isso limited so as to be undetectable by conventional means. The words“substantially” and “essentially” are used similarly as intensifiers inother places within this specification.

Referring to FIG. 1, there is provided a schematic representation of anelectrolytic cell 10 that can be used in the methods for recyclingalkali ions and producing alkali hydroxides according to the presentinvention described herein. In one embodiment, electrolytic cell 10 isused to make aqueous solutions of sodium hydroxide. The electrolyticcell 10 includes a container or shell 12, which may be corrosionresistant. An alkali-conducting ceramic membrane 14, which may bepositioned in or supported by a scaffold or holder 16, together with thecontainer 12 defines a catholyte compartment 18, and an anolytecompartment 20. The anolyte compartment 20 is configured with an anode22 and the catholyte compartment 18 is configured with a cathode 24. Anelectric potential or voltage source 25 is provided to operate theelectrolytic cell 10.

The container 12, and other parts of the cell 10, may be made of anysuitable material, including metal, glass, plastics, composite, ceramic,other materials, or combinations of the foregoing. The material thatforms any portion of the electrolytic cell 10 is preferably not reactivewith or substantially degraded by the chemicals and conditions that itis exposed to as part of the process.

The electrolytic cell 10 further comprises an anolyte inlet 26 forintroducing chemicals into the anolyte compartment 20 and an anolyteoutlet 28 for removing or receiving anolyte solution from the anolytecompartment 20. The cell 10 also includes a catholyte inlet 30 forintroducing chemicals into the catholyte compartment 18 and a catholyteoutlet 32 for removing or receiving catholyte solution from thecatholyte compartment 18. It will be appreciated by those of skill inthe art that the cell configuration and relative positions of the inletsand outlets may vary while still practicing the teachings of theinvention.

Because gases may be evolved from the cell during operation, ventingmeans (34, 36) are provided to vent, treat and/or collect gases from theanolyte compartment 20 and/or catholyte compartment 18. The means may bea simple venting system such as openings, pores, holes, and the like.The venting means may also include without limitation, a collectiontube, hose, or conduit in fluid communication with an airspace or gapabove the fluid level in the anolyte and/or catholyte compartments. Thegases which are evolved may be collected, vented to outside theelectrolytic cell, sent through a scrubber or other treatment apparatus,or treated in any other suitable manner.

The anode 22 and cathode 24 materials may be good electrical conductorsstable in the media to which they are exposed. Any suitable electricallyand catalytically active material may be used, and the material may besolid, plated, perforated, expanded, or the like. In one embodiment, theanode 22 and cathode 24 material is a dimensionally stable anode (DSA)which is comprised of ruthenium oxide coated titanium (RuO₂/Ti).Suitable anodes 22 can also be formed from nickel, cobalt, nickeltungstate, nickel titanate, platinum and other noble anode metals, assolids plated on a substrate, such as platinum-plated titanium. Kovar(Ni—Co—Fe), stainless steel, lead, graphite, tungsten carbide andtitanium diboride are also useful anode materials. Suitable cathodes 24may be formed from metals such as nickel, cobalt, platinum, silver,Kovar and the like. The cathodes 24 may also be formed from alloys suchas titanium carbide with small amounts of nickel. In one embodiment, thecathode is made of titanium carbide with less than about 3% nickel.Other embodiments include cathodes that include FeAl₃, NiAl₃, stainlesssteel, perovskite ceramics, and the like. Graphite is also a usefulcathode material. In some embodiments, the electrodes are chosen tomaximize cost efficiency effectiveness, by balancing electricalefficiency with low cost of electrodes.

The electrode material may be in any suitable form within the scope ofthe present invention, as would be understood by one of ordinary skillin the art. In some specific embodiments, the form of the electrodematerials may include at least one of the following: a solid, dense orporous solid-form, a dense or porous layer plated onto a substrate, aperforated form, an expanded form including a mesh, or any combinationthereof.

In some embodiments of the present invention, the electrode materialsmay be composites of electrode materials with non-electrode materials,where non-electrode materials are poor electrical conductors under theconditions of use. A variety of insulative non-electrode materials arealso known in the art, as would be understood by one of ordinary skillin the art. In some specific embodiments, the non-electrode materialsmay include at least one of the following: ceramic materials, polymers,and/or plastics. These non-electrode materials may also be selected tobe stable in the media to which they are intended to be exposed.

In some embodiments, only electrolytic reactions occur in the cell andgalvanic reactions are eliminated or greatly minimized. Accordingly,alkali ion conducting ceramic membranes 14 may include those whicheliminate or minimize galvanic reactions and promote only electrolyticreactions. In one embodiment, the membrane 14 has high ionicconductivity with minimal or negligible electronic conductivity. Themembrane may have high selectivity to preferred ionic species, such aslithium ions, sodium ions, or potassium ions. The membrane 14 may alsophysically separate the anolyte compartment from the catholytecompartment. This may be accomplished using a dense ceramic electrolyte.

The alkali ion conducting ceramic membrane 14 selectively transports aparticular, desired alkali metal cation species from the anolyte to thecatholyte side even in the presence of other cation species. The alkaliion conducting ceramic membrane 14 may also be impermeable to waterand/or other undesired metal cations. In some specific embodiments, theceramic membrane 14 has a current density from about 20 to about 200mA/cm². In one embodiment, the current through the alkali ion conductingceramic is predominately ionic current.

In some specific embodiments, the alkali ion conducting ceramicmembranes 14 are essentially impermeable to at least the watercomponents of both the first or catholyte solution and second or anolytesolution. These alkali ion conducting ceramic solid electrolytes orceramic membranes 14 may have low or even negligible electronicconductivity, which virtually eliminates any galvanic reactions fromoccurring when an applied potential or current is removed from the cellcontaining the membrane 14. In another embodiment, these alkali ionconducting ceramic solid electrolyte or ceramic membranes 14 areselective to a specific alkali metal ion and hence a high transferencenumber of preferred species, implying very low efficiency loss due tonear zero electro-osmotic transport of water molecules.

A variety of alkali ionalkali ion conducting ceramic materials are knownin the art and would be suitable for constructing the alkali ionalkaliion conducting solid electrolyte 14 of the present invention, as wouldbe understood by one of ordinary skill in the art. In an embodimentwithin the scope of the present invention, the alkali ion conductingceramic membrane 14 compositions comprising NaSICON (Sodium Super IonicConductors) materials are utilized for their characteristics of highion-conductivity for sodium ions at low temperatures, selectivity forsodium ions, current efficiency and chemical stability in water, ionicsolvents, and corrosive alkali media under static and electrochemicalconditions. Other similar alkali ion conducting ceramic membranes may behighly conductive for other alkali cations, such as lithium ions orpotassium ions. Such alkali ion conducting ceramic membranes 14 may haveone or more, or all, of the following desirable characteristics whichmake them suitable for aqueous and non-aqueous electrochemicalapplications. One characteristic is that, being dense, the ceramicmembrane 14 is at least substantially impervious to water transport, andis not influenced by scaling or precipitation of divalent ions,trivalent ions, and tetravalent ions or dissolved solids present in thesolutions. The ceramic membrane 14 may selectively transport sodium ionsin the presence of other ions at a transfer efficiency that is in someinstances above 90%. In some embodiments of the alkali cation-conductiveceramic materials of the present invention, the alkali cation-conductiveceramic materials may have a sodium ion conductivity ranging from about1×10⁻⁴ S/cm to about 5×10⁻¹ S/cm measured from ambient temperature toabout 85° C. In yet another embodiment the ceramic membrane 14 providesresistance to fouling by precipitants, and/or electro-osmotic transportof water, which is common with organic or polymer membranes.

In some specific embodiments, the alkali cation-conductive ceramicmembrane may include at least one of the following features and usecharacteristics, as would be understood by one of ordinary skill in theart: a solid form; a high alkali ion conductivity at temperatures belowabout 200° C.; low electronic conductivity; an alkali ion transferefficiency (i.e. high transference number) greater than about 95%; ahigh selectivity for particular alkali cations (e.g. Na⁺) in relation toother alkali or non-alkali cations; stability in solutions of alkali ioncontaining salts and chemicals of weak or strong organic or inorganicacids; a density greater than about 95% of theoretical density value;substantially impermeable to water transport; resistant to acid,alkaline, caustic and/or corrosive chemicals.

As noted above, in some specific embodiments, the cation conducted bythe alkali ion conducting ceramic membrane 14 is the sodium ion (Na⁺).In some specific embodiments, sodium-ion conducting ceramic membranesinclude alkali ion conducting ceramic membrane 14 compositionscomprising NaSICON-type materials of general formulaNa_(1+x)Zr₂Si_(x)P_(3−x)O₁₂ where 0≦x≦3. The membrane 14 may includeNaSICON materials of general formula Na₅RESi₄O₁₂ and non-stoichiometricsodium-deficient materials of general formula(Na₅RESi₄O₁₂)_(1−δ)(RE₂O₃.2SiO₂)_(δ), where RE is Nd, Dy, or Sm, or anymixture thereof and where δ is the measure of deviation fromstoichiometry, as disclosed in U.S. Pat. No. 5,580,430, and asexplicitly incorporated herein by this reference in its entirety. Otheranalogs of NaSICON materials to transport alkali ions such as Li and K,to produce other alkali hydroxides are known to those of ordinary skillin the art, and their use is encompassed within the scope of thisinvention. These alkali ion conducting ceramic membranes comprisingNaSICON materials or comprising analogs of NaSICON materials areparticularly useful in electrolytic systems for simultaneous productionof alkali hydroxides, by electrolysis of alkali (e.g., sodium) saltsolutions. In specific methods, a solid sodium-ion conducting ceramicmembrane 14 separates two compartments of a cell. The sodium ionstransfer across the membrane 14 from the anolyte to the catholytecompartment under the influence of electrical potential to generatesodium hydroxides or mixture of sodium salts or react to combine withother in-organic or organic compounds. Certain alkali ion conductingmembranes do not allow transport of water there through, which makes theprocess more energy efficient. Furthermore, these ceramic membranes havelow electronic conductivity, superior corrosion resistance, and highflux of specific alkali ions providing high ionic conductivity.

In some specific embodiments, the alkali ion conducting ceramic membrane14 comprises NaSICON-type materials that may include at least one of thefollowing: materials of general formula M_(1+x)M^(I) ₂Si_(x)P_(3−x)O₁₂where 0≦x≦3, where M is selected from the group consisting of Li, Na, K,or Ag, or mixture thereof, and where M^(I) is selected from the groupconsisting of Zr, Ge, Ti, Sn, Y or Hf, or mixtures thereof; materials ofgeneral formula Na_(1+z)L_(z)Zr_(2−z)P₃O₁₂ where 0≦z≦2.0, and where L isselected from the group consisting of Cr, Yb, Er, Dy, Sc, Fe, In, or Y,or mixtures or combinations thereof; materials of general formula M^(II)₅RESi₄O₁₂, where M^(II) may be Li, Na, K or Ag, or any mixture orcombination thereof, and where RE is Y or any rare earth element.

In some specific embodiments, the NaSICON-type materials may include atleast one of the following: non-stoichiometric materials,zirconium-deficient (or sodium rich) materials of general formulaNa_(1+x)Zr_(2−x/3)Si_(x)P_(3−x)O_(12−2x/3) where 1.55≦x≦3. In somespecific embodiments, the alkali ion conducting ceramic membrane 14compositions comprising NaSICON-type materials may include at least oneof the following: non-stoichiometric materials, sodium-deficientmaterials of general formulaNa_(1+x)(A_(y)Zr_(2−y))(Si_(z)P_(3−z))O_(12−δ) where A is selected fromthe group consisting of Yb, Er, Dy, Sc, In, or Y, or mixtures orcombinations thereof, 1.8≦x≦2.6, 0≦y≦0.2, x<z, and δ is selected tomaintain charge neutrality. In some specific embodiments, theNASICON-type materials may include sodium-deficient materials of formulaNa_(3.1)Zr₂Si_(2.3)P_(0.7)O_(12−δ).

Other exemplary NaSICON-type materials are described by H. Y—P. Hong in“Crystal structures and crystal chemistry in the systemNa_(1+x)Zr₂Si_(x)P_(3−x)O₁₂”, Materials Research Bulletin, Vol. 11, pp.173-182, 1976; J. B. Goodenough et al., in “Fast Na⁺-ion transportskeleton structures”, Materials Research Bulletin, Vol. 11, pp. 203-220,1976; J. J. Bentzen et al., in “The preparation and characterization ofdense, highly conductive Na₅GdSi₄O₁₂ nasicon (NGS)”, Materials ResearchBulletin, Vol. 15, pp. 1737-1745, 1980; C. Delmas et al., in “Crystalchemistry of the Na_(1+x)Zr_(2−x)L_(x)(PO₄)₃ (L=Cr, In, Yb) solidsolutions”, Materials Research Bulletin, Vol. 16, pp. 285-290, 1981; V.von Alpen et al., in “Compositional dependence of the electrochemicaland structural parameters in the NASICON system(Na_(1+x)Si_(x)Zr₂P_(3−x)O₁₂)”, Solid State Ionics, Vol. 3/4, pp.215-218, 1981; S. Fujitsu et al., in “Conduction paths in sintered ionicconductive material Na_(1+x)Y_(x)Zr_(2−x)(PO₄)₃”, Materials ResearchBulletin, Vol. 16, pp. 1299-1309, 1981; Y. Saito et al., in “Ionicconductivity of NASICON-type conductors Na_(1.5)M_(0.5)Zr_(1.5)(PO₄)₃(M: Al³⁺, Ga³⁺, Cr³⁺, Sc³⁺, Fe³⁺, In³⁺, Yb³⁺, Y³⁺)”, Solid State Ionics,Vol. 58, pp. 327-331, 1992; J. Alamo in “Chemistry and properties ofsolids with the [NZP] skeleton”, Solid State Ionics, Vol. 63-65, pp.547-561, 1993; K. Shimazu in “Electrical conductivity and Ti⁴⁺ ionsubstitution range in NASICON system”, Solid State Ionics, Vol. 79, pp.106-110, 1995; Y. Miyajima in “Ionic conductivity of NASICON-typeNa_(1+x)M_(x)Zr_(2−x)P₃O₁₂ (M: Yb, Er, Dy)”, Solid State Ionics, Vol.84, pp. 61-64, 1996. These references are incorporated in their entiretyherein by this reference.

While the alkali ion conducting ceramic materials disclosed hereinencompass or include many formulations of NaSICON materials, thisdisclosure concentrates on an examination of ceramic membranescomprising NaSICON materials for the sake of simplicity. The focuseddiscussion of NaSICON materials as one example of materials is not,however, intended to limit the scope of the invention. For example, thematerials disclosed herein as being highly conductive and having highselectivity include those alkali super ion conducting materials that arecapable of transporting or conducting any alkali cation, such as sodium(Na), lithium (Li), potassium (K), ions for producing alkali hydroxides.

The alkali ion conducting ceramic membranes comprising NaSICON materialsmay be used or produced for use in the processes and apparatus of thepresent invention in any suitable form, as would be understood by one ofordinary skill in the art. In some specific embodiments, the form of thealkali ion conducting ceramic membranes may include at least one of thefollowing: monolithic flat plate geometries, supported structures inflat plate geometries, monolithic tubular geometries, supportedstructures in tubular geometries, monolithic honeycomb geometries, orsupported structures in honeycomb geometries. In another embodiment, themembrane 14 may be a supported membrane 14 known to those of skill inthe art. Supported structures or membranes may comprise dense layers ofion-conducting ceramic solid electrolyte supported on porous supports. Avariety of forms for the supported membranes are known in the art andwould be suitable for providing the supported membranes for alkali ionconducting ceramic membranes with supported structures, including, butnot limited to: ceramic layers sintered to below full density withresultant continuous open porosity, slotted-form layers, perforated-formlayers, expanded-form layers including a mesh, or combinations thereof.In some embodiments, the porosity of the porous supports issubstantially continuous open-porosity so that the liquid solutions oneither side of the alkali ion conducting ceramic membrane 14 may be inintimate contact with a large area of the dense-layers of alkali ionconducting ceramic solid electrolytes, and in some, the continuousopen-porosity ranges from about 30 volume % to about 90 volume %. Insome embodiments of the present invention, the porous supports for thesupported structures may be present on one side of the dense layer ofalkali ion conducting ceramic solid electrolyte. In some embodiments ofthe present invention, the porous supports for the supported structuresmay be present on both sides of the dense layer of alkali ion conductingceramic solid electrolyte.

A variety of materials for the porous supports or supported membranesare known in the art and would be suitable for providing the poroussupports for alkali ion conducting ceramic membranes withsupported-structures, including: electrode materials, NaSICON-typematerials, β^(I)-alumina, β^(II)-alumina, other ion-conducting ceramicsolid electrolyte materials, and non-conductive materials such asplastics, polymers, organics or ceramic materials, metals, and metalalloys. The thickness of the dense layer of alkali ion conductingceramic solid electrolyte material in monolithic structures is generallyfrom about 0.3 mm to about 5 mm, and in some instances from about 0.5 mmto about 1.5 mm. The thickness of the dense layer of alkali ionconducting ceramic solid electrolyte material in supported-structures isgenerally from about 25 μm to about 2 mm, and often from about 0.5 mm toabout 1.5 mm. Layers as thin as about 25 μm to about 0.5 mm are readilyproducible, as would be understood by one of ordinary skill in the art.In some specific embodiments, the alkali ion conducting ceramicmembranes are structurally supported by the cathode, which is porous.This may dictate characteristics of both the form of the alkali ionconducting ceramic membranes, and/or of the cathode and/or anode. Insome specific embodiments, the porous substrate has similar thermalexpansion and good bonding with the alkali ion conducting ceramicmembrane 14 as well as good mechanical strength. One of ordinary skillin the art would understand that the number and configuration of thelayers used to construct the alkali ion conducting ceramic membrane 14as supported-structures could be widely varied within the scope of theinvention.

In some embodiments of the alkali ion conducting ceramic membranes ofthe present invention, the alkali ion conducting ceramic membranes maybe composites of alkali ion conducting ceramic solid electrolytematerials with non-conductive materials, where the non-conductivematerials are poor ionic and electronic electrical conductors under theconditions of use. A variety of insulative non-conductive materials arealso known in the art, as would be understood by one of ordinary skillin the art. In some specific embodiments, the non-conductive materialsmay include at least one of the following: ceramic materials, polymers,and/or plastics that are substantially stable in the media to which theyare exposed.

Layered alkali ion conducting ceramic-polymer composite membranes arealso particularly suitable for use as alkali ion conducting ceramicmembranes in the present invention. Layered alkali ion conductingceramic-polymer composite membranes generally comprise ion-selectivepolymers layered on alkali ion conducting ceramic solid electrolytematerials. In some specific embodiments, the alkali ion conductingceramic solid electrolyte materials of the layered alkali ion conductingceramic-polymer composite membranes may include at least one of thefollowing: NaSICON-type materials or beta-alumina. Ion-selective polymermaterials have the disadvantage of having poor selectively to sodiumions, yet they demonstrate the advantage of high chemical stability.

In some specific embodiments, the alkali ion conducting ceramic membrane14 may comprise two or more co-joined layers of different alkali ionconducting ceramic membrane 14 materials. Such co-joined alkali ionconducting ceramic membrane 14 layers could include NaSICON materialsjoined to other alkali ion conducting ceramic materials, such as, butnot limited to, beta-alumina. Such co-joined layers could be joined toeach other using a method such as, but not limited to, thermal spraying,plasma spraying, co-firing, joining following sintering, etc. Othersuitable joining methods are known by one of ordinary skill in the artand are included herein.

The alkali ion conducting ceramic solid electrolyte materials disclosedherein are particularly suitable for use in the electrolysis of alkalimetal based salt solutions because they have high ion-conductivity foralkali metal cations at low temperatures, high selectivity for alkalimetal cations, good current efficiency and stability in water andcorrosive media under static and electrochemical conditions.Comparatively, beta alumina is a ceramic material with high ionconductivity at temperatures above 300° C., but has low conductivity attemperatures below 100° C., making it less practical for applicationsbelow 100° C.

Sodium ion conductivity in NaSICON structures has an Arrheniusdependency on temperature, generally increases as a function oftemperature. The sodium ion conductivity of ceramic membranes comprisingNaSICON materials ranges from about 1×10⁻⁴ S/cm to about 1×10⁻¹ S/cmfrom room temperature to 85° C.

Alkali ion conducting ceramic membranes comprising NaSICON materials,especially of the type described herein, have low or negligibleelectronic conductivity, and as such aid in virtually eliminating theoccurrence of any galvanic reactions when the applied potential orcurrent is removed. Certain NaSICON analogs according to the presentinvention have very mobile cations, including, but not limited tolithium, sodium, and potassium ions, that provide high ionicconductivity, low electronic conductivity and comparatively highcorrosion resistance.

The sodium-ion conducting ceramic materials referred herein for use inelectrolytic cells can be used successfully in the formation of sodiumhydroxides from the electrolysis of aqueous sodium salt solutions,including, but not limited to, such solutions as sodium carbonate,sodium nitrate, sodium phosphate, sodium hypochlorite, sodium chloride,sodium perchlorate, and sodium organic salts.

One alkali ion conducting ceramic solid electrolyte or alkali ionconducting ceramic membrane 14 is an electronic insulator and anexcellent ionic conductor. The Na_(1+x)Zr₂Si_(x)P_(3−x)O₁₂ (where 0≦x≦3)composition is the best known member of a large family of sodium-ionconducting ceramic solid electrolyte materials that have beenextensively studied. The structure has hexagonal arrangement and remainsstable through a wide variation in atomic parameters as well in thenumber of extra occupancies or vacancies.

One of ordinary skill in the art would understand that a number ofceramic powder processing methods are known for processing of the alkaliion conducting ceramic solid electrolyte materials such as hightemperature solid-state reaction processes, co-precipitation processes,hydrothermal processes, or sol-gel processes. In some embodiments of thepresent invention it may be advantageous to synthesize the alkali ionconducting ceramic solid electrolyte materials by high temperaturesolid-state reaction processes. Specifically, ceramic the processing ofNa_(1+x)Zr₂Si_(x)P_(3−x)O₁₂ (where 0≦x≦3) and Na₅RESi₄O₁₂ NaSICONcompositions (where RE is either Yttrium or a rare earth element) mayproceed as follows. Alkali ion conducting ceramic membranes may besystematically synthesized by solid-state oxide mixing techniques. Amixture of the starting precursors may be mixed in methanol inpolyethylene jars, and the mixed precursor oxides are dried at 60° C. toevolve the solvent. The dried powder or material may be calcined at 800°C., to form the required composition, followed by wet ball milled withzirconium oxide media (or another media known to one of ordinary skillin the art) to achieve the prerequisite particle size distribution. Oneof ordinary skill in the art would understand that a number of polymersare known for processing with ceramic powders such as those set forthabove as prerequisite for preparing a green-form, and that a number ofconventional ceramic fabrication processing methods are known forprocessing ceramic membranes such as those set forth above in agreen-form. Green-form membranes at 0.60 to 2.5 inch diameter sizes maybe pressed by compaction in a die and punch assembly and then sinteredin air at temperatures between 1100° C. and 1200° C. to make densealkali ion conducting ceramic membranes. XRD analysis of the alkali ionconducting ceramic membranes may be performed to identify the NaSICONcomposition crystal structure and phase purity. Stoichiometric andnon-stoichiometric compositions of the Na_(1+x)Zr₂Si_(x)P_(3−x)O₁₂ typeformula (where 0≦x≦3) are one type of alkali ion conducting ceramicmembrane 14 produced in this manner. Non-stoichiometric in this instancemeans un-equivalent substitution of Zr, Si, and/or P in the formula. Thestability or resistance to corrosive media of the alkali ion conductingceramic membrane 14 materials described herein may be enhanced bychemistry variation

The alkali ion conducting ceramic membrane 14 may have flat plategeometry, tubular geometry, or supported geometry. The solid membrane 14may be sandwiched between two pockets, made of a chemically-resistantHDPE, PPE, PPR plastics and sealed, by compression loading using asuitable gasket or o-ring, such as an EPDM (ethylene propylene dienemonomer) rubber gaskets or o-ring.

The NaSICON materials or modified NaSICON materials referred herein areuseful, for example, as sodium-ion conducting ceramic membranes inelectrolytic cells. In one embodiment, the method for the production ofsodium hydroxide solution comprises introducing a lower concentrationsolution of sodium hydroxide solution into a catholyte compartment of anelectrolytic cell, introducing an aqueous solution comprising one ormore sodium salts (examples of stream chemistry presented in Tables 1and 2) into an anolyte compartment of the electrolytic cell, wherein theanolyte compartment and the catholyte compartment of the electrolyticcell are separated by a ceramic membrane 14 comprising NaSICON, applyingelectric potential across the electrodes in the electrolytic cell toselectively transport sodium ions from the anolyte compartment to thecatholyte compartment where the sodium ions react with the hydroxyl ionsto form sodium hydroxide at higher concentration in the catholytecompartment of the electrolytic cell, and wherein the composition of thesolution of sodium hydroxide solution in the catholyte compartment ofthe electrolytic cell comprises between at least about 2% by weightsodium hydroxide and at most about 50% by weight sodium hydroxide. In afurther embodiment, the method comprises separation of the anolytecompartment and the catholyte compartment of the electrolytic cell by aceramic membrane 14 comprising NaSICON, and a composition of thesolution of sodium hydroxide in the catholyte compartment of theelectrolytic cell comprising between at least about 1% by weight sodiumhydroxide and at most about 30% by weight sodium hydroxide. In a furtherembodiment, the method comprises separation of the anolyte compartmentand the catholyte compartment of the electrolytic cell by a ceramicmembrane 14 comprising NaSICON, and a composition of the solution ofsodium hydroxide solution in the catholyte compartment of theelectrolytic cell comprising between at least about 0.1% by weightsodium hydroxide and at most about 20% by weight sodium hydroxide.

An example of an overall electrolytic reaction, using sodium hydroxideas the source of sodium ion, is as follows:

Anode: 2OH⁻→½O₂+H₂O+2e ⁻

Cathode: 2H₂O+2e ⁻

2OH⁻+H₂

2Na⁺+2OH⁻

2NaOH

An example of an overall electrolytic reaction, using sodium salts in anaqueous waste stream as the source of sodium ion, is as follows:

Anode: 2H₂O→O₂+4H⁺+4e ⁻

Cathode: 2H₂O+2e ⁻

2OH⁻+H₂

2Na⁺+20H⁻

2NaOH

The reactions described above are electrolytic reactions, taking placeunder an induced current wherein electrons are introduced or are removedto cause the reactions. The reactions proceed only so long as a currentis flowing through the cell. Contrary to electrolytic reactions,galvanic reactions may occur when an applied potential to the cell isremoved, which tends to reduce the efficiency of the electrolytic cell.In one embodiment, only electrolytic reactions occur in the cell andgalvanic reactions are eliminated or greatly minimized.

In some specific embodiments, the alkali cation-conductive ceramicmembrane may comprise two or more co-joined layers of different alkalication-conductive ceramic materials. Such co-joined alkalication-conductive ceramic membrane layers could include NaSICON-typematerials joined to other ceramics, such as, but not limited to,beta-alumina. Such layers could be joined to each other using a methodsuch as, but not limited to, co-firing, joining following sintering,etc. Other suitable joining methods are known by one of ordinary skillin the art and are included herein.

The alkali cation-conductive ceramic membranes may be used or producedfor use in the processes and apparatus of the present invention in anysuitable form, as would be understood by one of ordinary skill in theart. In some specific embodiments, the form of the alkalication-conductive ceramic membranes may include at least one of thefollowing: monolithic flat plate geometries, supported structures inflat plate geometries, monolithic tubular geometries, supportedstructures in tubular geometries, monolithic honeycomb geometries, orsupported structures in honeycomb geometries. Supported structures maycomprise dense layers of alkali cation-conductive ceramic materialssupported on porous supports. A variety of forms for the porous supportsare known in the art and would be suitable for providing the poroussupports for alkali cation-conductive ceramic membranes with supportedstructures, including: ceramic layers sintered to below full densitywith resultant continuous open porosity, slotted-form layers,perforated-form layers, expanded-form layers including a mesh, orcombinations thereof. In some embodiments, the porosity of the poroussupports is substantially continuous open-porosity so that the liquidsolutions on either side of the alkali cation-conductive ceramicmembrane may be in intimate contact with a large area of thedense-layers of alkali cation-conductive ceramic materials, and in some,the continuous open-porosity ranges from about 30 volume % to about 90volume %. In some embodiments of the present invention, the poroussupports for the supported structures may be present on one side of thedense layer of alkali cation-conductive ceramic material. In someembodiments of the present invention, the porous supports for thesupported structures may be present on both sides of the dense layer ofalkali cation-conductive ceramic material.

A variety of materials for the porous supports are known in the art andwould be suitable for providing the porous supports for alkalication-conductive ceramic membranes with supported-structures,including: electrode materials, NaSICON-type materials, β^(I)-alumina,β^(II)-alumina, other cation-conductive materials, and non-conductivematerials such as plastics or ceramic materials, metals, and metalalloys. The thickness of the dense layer of alkali cation-conductiveceramic material in monolithic structures is generally from about 0.3 mmto about 5 mm, and in some instances from about 0.5 mm to about 1.5 mm.The thickness of the dense layer of alkali cation-conductive ceramicmaterial in supported-structures is generally from about 25 μm to about2 mm, and often from about 0.5 mm to about 1.5 mm. Layers as thin asabout 25 μm to about 0.5 mm are readily producible, as would beunderstood by one of ordinary skill in the art. In some specificembodiments, the alkali cation-conductive ceramic membranes arestructurally supported by the cathode, which is porous. This may dictatecharacteristics of both the form of the membranes, and/or of the cathodeand/or anode. In some specific embodiments, the porous substrate musthave similar thermal expansion and good bonding with the alkalication-conductive ceramic membrane as well as good mechanical strength.One of ordinary skill in the art would understand that the number andconfiguration of the layers used to construct the alkalication-conductive ceramic membrane as supported-structures could bewidely varied within the scope of the invention.

In some embodiments of the alkali cation-conductive ceramic membranes ofthe present invention, the alkali cation-conductive ceramic membranesmay be composites of alkali cation-conductive ceramic materials withnon-conductive materials, where the non-conductive materials are poorionic and electronic electrical conductors under the conditions of use.A variety of insulative non-conductive materials are also known in theart, as would be understood by one of ordinary skill in the art. In somespecific embodiments, the non-conductive materials may include at leastone of the following: ceramic materials, polymers, and/or plastics thatare substantially stable in the media to which they are exposed.

It not necessary for the cathode to contact the alkali cation-conductiveceramic membrane in the processes or apparatus of the present invention.Both the catholyte and anolyte are ion-conductive so that from anelectrical standpoint the electrodes may be remote from the membranes.In such an event, a thin-film dense alkali cation-conductive ceramicmembrane may be deposited on a porous substrate which does not have tobe an electrode,

One of ordinary skill in the art would understand that a number ofceramic powder processing methods are known for processing of the alkalication-conductive ceramic materials such as high temperature solid-statereaction processes, co-precipitation processes, hydrothermal processes,or sol-gel processes. In some embodiments of the present invention itmay be advantageous to synthesize the alkali cation-conductive ceramicmaterials by high temperature solid-state reaction processes.Specifically, for NaSICON-type materials, a mixture of startingprecursors such as simple oxides and/or carbonates of the individualcomponents may be mixed at the desired proportions in methanol inpolyethylene vessels, and dried at approximately 60° C. to evolve thesolvent; the dried mixture of starting precursors may be calcined in therange of from about 800° C. to about 1200° C. dependent on thecomposition, followed by milling of the calcined powder with media suchas stabilized-zirconia or alumina or another media known to one ofordinary skill in the art to achieve the prerequisite particle sizedistribution. To achieve the prerequisite particle size distribution,the calcined powder may be milled using a technique such as vibratorymilling, attrition milling, jet milling, ball milling, or anothertechnique known to one of ordinary skill in the art, using media (asappropriate) such as stabilized-zirconia or alumina or another mediaknown to one of ordinary skill in the art.

One of ordinary skill in the art would understand that a number ofconventional ceramic fabrication processing methods are known forprocessing ceramic membranes such as those set forth above in agreen-form. Such methods include, but are not limited to, tape casting,calendaring, embossing, punching, laser-cutting, solvent bonding,lamination, heat lamination, extrusion, co-extrusion, centrifugalcasting, slip casting, gel casting, die casting, pressing, isostaticpressing, hot isostatic pressing, uniaxial pressing, and sol gelprocessing. The resulting green form ceramic membrane may then besintered to form an alkali cation-conductive ceramic membrane using atechnique known to one of ordinary skill in the art, such asconventional thermal processing in air, or controlled atmospheres tominimize loss of individual components of the alkali cation-conductiveceramic membranes. In some embodiments of the present invention it maybe advantageous to fabricate the ceramic membrane in a green form bydie-pressing, optionally followed by isostatic pressing. In otherembodiments of the present invention it may potentially be advantageousto fabricate the ceramic membrane as a multi-channel device in a greenform using a combination of techniques such as tape casting, punching,laser-cutting, solvent bonding, heat lamination, or other techniquesknown to one of ordinary skill in the art. Specifically, forNaSICON-type materials, a ceramic membrane in a green-form may begreen-formed by pressing in a die, followed by isostatic pressing andthen sintering in air in the range of from about 925° C. to about 1300°C. for up to about 8 hours to make sintered alkali cation-conductiveceramic membrane structures with dense layers of alkalication-conductive ceramic materials. Standard x-ray diffraction analysistechniques may be performed to identify the crystal structure and phasepurity of the alkali cation-conductive ceramic materials in the sinteredceramic membrane.

In some specific embodiments, alkali cation-conductive ceramic membranesfor use in the processes and apparatus of the present invention may befabricated by a vapor deposition method onto a porous support, includingat least one of the following methods: physical vapor deposition,chemical vapor deposition, sputtering, thermal spraying, or plasmaspraying. The thickness of the alkali cation-conductive ceramic membraneformed by a vapor deposition method onto a porous support is generallyfrom about 1 μm to about 100 μm, but may be varied as is known to one ofordinary skill in the art.

In one embodiment of the processes and apparatus of the presentinvention, the electrolytic cell 10 may be operated in a continuousmode. In a continuous mode, the cell is initially filled with anolytesolution and catholyte solution and then, during operation, additionalsolutions are fed into the cell and products, by-products, and/ordiluted solutions are removed from the cell without ceasing operation ofthe cell. The feeding of the anolyte solution and catholyte solution maybe done continuously or it may be done intermittently, meaning that theflow of a given solution is initiated or stopped according to the needfor the solution and/or to maintain desired concentrations of solutionsin the cell compartments, without emptying any one individualcompartment or any combination of the two compartments. Similarly, theremoval of solutions from the anolyte compartment and the catholytecompartment may also be continuous or intermittent. Control of theaddition and/or removal of solutions from the cell may be done by anysuitable means. Such means include manual operation, such as by one ormore human operators, and automated operation, such as by using sensors,electronic valves, laboratory robots, etc. operating under computer oranalog control. In automated operation, a valve or stopcock may beopened or closed according to a signal received from a computer orelectronic controller on the basis of a timer, the output of a sensor,or other means. Examples of automated systems are well known in the art.Some combination of manual and automated operation may also be used.Alternatively, the amount of each solution that is to be added orremoved per unit time to maintain a steady state may be experimentallydetermined for a given cell, and the flow of solutions into and out ofthe system set accordingly to achieve the steady state flow conditions.

In another embodiment, the electrolytic cell 10 is operated in batchmode. In batch mode, the anolyte solution and catholyte solution are fedinitially into the cell and then the cell is operated until the desiredconcentration of product is produced in the anolyte and catholyte. Thecell is then emptied, the products collected, and the cell refilled tostart the process again. Alternatively, combinations of continuous modeand batch mode production may be used. Also, in either mode, the feedingof solutions may be done using a pre-prepared solution or usingcomponents that form the solution in situ.

It should be noted that both continuous and batch mode have dynamic flowof solutions. In one embodiment of continuous mode operation, theanolyte solution is added to the anolyte chamber so that the sodiumconcentration is maintained at a certain concentration or concentrationrange during operation of the electrolytic cell 10. In one embodiment ofbatch mode operation, a certain quantity of sodium ions are transferredthrough the alkali cation-conductive ceramic membrane to the catholytechamber and are not replenished, with the cell operation is stopped whenthe sodium concentration in the anolyte-chamber reduces to a certainamount or when the appropriate product concentration is reached in thecatholyte.

Several examples are provided below which discuss the construction, use,and testing of specific embodiments of the present invention. Theseembodiments are exemplary in nature and should not be construed to limitthe scope of the invention in any way.

A two-compartment electrolytic cell as described in FIG. 1, based on analkali cation-conductive ceramic material membrane 14 is used to splitan aqueous solution of sodium salts based complex chemistry. The alkalication-conductive ceramic material 14 may be NaSICON-type material. Thealkali ions transport across the alkali cation-conductive ceramicmembrane to the catholyte compartment 18. The gaseous product is oxygenin the anolyte compartment 20 venting at 34, and hydrogen in thecatholyte compartment 18 venting at 36. Reactions are as follows:

Anolyte compartment: H₂O→2H⁺+2e ⁻ +½O ₂

Catholyte compartment: 2H₂O+2e ⁻→H₂+2OH⁻

The anolyte compartment 20 is greater than about pH 7 and below about pH14, and whereas the catholyte compartment 18 is greater than about pH 7.The process can be operated between about ambient temperature and about125° C. The anolyte inlet 26 is an aqueous feed of sodium based saltssuch as sodium hydroxide, sodium sulfate and other alkali and transitionmetal impurities. The catholyte inlet 30 is an aqueous solution ofsodium hydroxide. The catholyte outlet 32 is an aqueous solution ofsodium hydroxide, at a higher concentration than the catholyte inlet 30.

The radioactively-contaminated aqueous (nuclear) waste stream typicallycomprises significant amounts of sodium nitrate, sodium nitrite, sodiumhydroxide, sodium carbonate, sodium hydroxide, sodium chloride, sodiumchlorate, sodium oxalate, sodium fluoride and salts of potassium,cesium, and strontium, calcium, aluminum, and host of radionuclideelements (Cs, Sr). The inlet stream of radioactively-contaminatedaqueous (nuclear) waste comprising sodium hydroxide, strontium nitrate,sodium chloride, sodium fluoride, sodium hydrogen phosphate, sodiumcarbonate, sodium oxalate, sodium sulfate, sodium nitrite, sodiumnitrate, potassium nitrate, potassium hydroxide, cesium nitrate, calciumnitrate, cesium nitrate, barium nitrate, silicon dioxide, and aluminumnitrate, iron oxide, iron nitrate, chromium oxide, and a list of otherelements in the periodic table.

Example 1

A two-compartment electrolytic cell as described in FIG. 1, based on analkali cation-conductive ceramic membrane 14 (NASD10 membrane-1.4 mmthick) was assembled. Platinum and nickel were used as anode andcathode. The alkali cation-conductive ceramic membrane material 14 isNa₃Zr₂Si₂PO₁₂ composition. The electrolytic cell was operated in batchmode in 3 molar NaNO₃+2 Molar NaOH solutions with fresh solutionperiodically replacing the sodium replenished anolyte. The cell wasoperated at 100 mA/cm² current density at 38° C. for 900 hours. Themembrane transported approximately 63 moles of sodium from the anolyteto the catholyte chamber. The voltage across the membrane was between 3and 6.5 volts. The gaseous products in oxygen in the anolyte compartment20 venting at 34, and hydrogen in the catholyte compartment 18 ventingat 36.

Example 2

A two-compartment electrolytic cell as described in FIG. 1, based on analkali cation-conductive ceramic membrane 14 was assembled. The alkalication-conductive ceramic membrane material 14 is Na₃Zr₂Si₂PO₁₂composition (coded: NAS-D10). Platinum and nickel were used as anode andcathode. The electrolytic cell was operated in batch mode in NaOHsolution, with fresh solution periodically replacing the replenishedanolyte. The cell was continuously operated at 4.5 V and 40° C. for 5000hours. FIG. 3 shows the test result, where the sodium ionic current iscompared to the total current. The sodium transport efficiency achievedis very high (>90%) and remains constant up to 3000 hours, and dropsbelow 90% between 3000-5000 hours of testing. The transport efficiencywas measured periodically when a fresh batch of anolyte was introduced.Though the membrane was operated at a relatively small current densityof 25 mA/cm², it is evident from this test that the sodium transportefficiency is very high and steady at 90%, and most importantly, thatthe NAS D10 membrane was structurally stable during the entire durationof the test. The gaseous products are oxygen in the anolyte compartment20 venting at 34, and hydrogen in the catholyte compartment 18 ventingat 36

Example 3

A two-compartment electrolytic cell as described in FIG. 1, based on analkali cation-conductive ceramic material 14 with four NASE (2 inchdiameter) membranes were assembled in the scaffold fitted into anElectrocell MP cell configuration. The alkali cation-conductive ceramicmaterial 14 is Na_(3.2)Zr₂Si_(2.2)P_(0.8)O₁₂ composition (coded: NAS-E).Platinum and nickel were used as anode and cathode. The stack waschecked by pressuring the anolyte inlet with DI water and looked forleak in the catholyte compartment. The leak checked scaffold wasassembled into the Electrocell MP, and secured in the testing jig. Theplumbing connection was made between the inlets and outlets of theElectrocell MP cell and the individual holding tanks, and similarlyelectrical connection was made between the electrodes, power supply andthe data acquisition system.

The 20 liter holding tanks were half filled with 1.5 molar sodium basedaqueous electrolytes, and heated to just above 40° C. and allowed toequilibrate. The pumps were turned on to circulate the electrolyte inthe cell manifold and checked for leaks before the power supply wasturned on. The flow rate was maintained at 1.6 gpm. The stack wasoperated in constant current mode at 100 mA/cm² current density. Thegaseous products are oxygen in the anolyte compartment 20 venting at 34,and hydrogen in the catholyte compartment 18 venting at 36. The cellvoltage remained steady during the entire duration of the test (FIG. 4).The sodium transport efficiency after every 150 hours of testing wasbetween 90 and 96%.

Example 4

A two-compartment electrolytic cell as described in FIG. 1, based on analkali cation-conductive ceramic material membrane 14 to synthesizesodium hydroxide and an acid from transition-metal based sodium saltscontaining by-product industrial stream. The alkali cation-conductiveceramic material 14 is Na_(3.3)Zr₂Si_(2.3)P_(0.7)O₁₂ composition (coded:NAS-G). Platinum and nickel were used as anode and cathode.

The electrolytic cell was operated using 1.5 M aqueous NaOH anolyte andcatholyte. The purpose of this test was to validate the influence ofvarious cations and anions present in the anolyte on electrochemicalperformance of the ceramic membrane. This test was operated at 50° C.,and current density in the 100 to 150 mA/cm² range. The results areshown in FIG. 5 in which the sodium current density is compared to totalcurrent efficiency. High sodium current efficiency (90%) was observedagain which remained constant during the 300 hours of testing. The totalapplied voltage in this test was high (8.5 to 9 volts). The steady stateconductivity of the NASG membrane during the test, based on the voltagemeasurement across the membrane, was 2×10⁻² S/cm. The sodium massbalance of anolyte and the catholyte in this experiment did not show anyloss of sodium

Example 5

A two-compartment electrolytic cell as described in FIG. 1, based on analkali cation-conductive ceramic material membrane 14 to synthesizesodium hydroxide from transition-metal based sodium salts containingby-product industrial stream was assembled. The alkali cation-conductiveceramic material 14 is Na_(3.3)Zr₂Si_(2.3)P_(0.7)O₁₂ composition (coded:NAS-G). Platinum and nickel were used as anode and cathode.

The electrolytic cell was operated using 1.5 M NaOH catholyte. Thecomposition of the anolyte was prepared to simulate the radioactivelycontaminated salt waste stream at Savannah River Site, a DOE facility.The composition of this anolyte feed (Simulant Chemistry 1) is shown inTable 1.

TABLE 1 Anolyte Simulant Chemistry 1 wt % H₂O 55.540% NaOH 8.252%Sr(NO₃)₂ 0.018% NaCl 0.092% NaF 0.066% Na₂HPO₄ 0.096% Na₂C₂O₄ 0.011%Na₂CO₃ 1.330% Na₂SO₄ 1.562% NaNO₂ 3.256% NaNO₃ 6.734% KNO₃ 0.056%Ca(NO₃)₂•4H₂O 0.018% CsNO₃ 0.002% SiO₂ 0.020% Al(NO₃)₃•9H₂O 22.947%

The purpose of this test was to validate the influence of variouscations and anions present in the anolyte on the electrochemicalperformance of the ceramic membrane. This test was operated at 50° C.,and constant current density of 200 mA/cm². The results are shown inFIG. 6. High sodium current efficiency (90%) was observed again whichremained constant during the 300 hours of testing. The total averagevoltage across the membrane was 4.75 volts because of the cell designwhere the electrodes were positioned far away from the membrane. Duringthe course of this test, Al(OH)₃ precipitated in the anolyte when the pHof the solution dropped below 12. This however did not influence theperformance of membrane. Sodium ions are selectively transported fromanolyte membrane across the membrane in the presence of other ⁺1, ⁺2,⁺3, ⁺4 and ⁺5 valence cations and anions such as NO₃ ⁻, CO₃ ²⁻, SO₄ ²⁻,NO²⁻, Cl⁻, F⁻, PO₄ ³⁻, and HPO₄ ²⁻ ions in the anolyte stream. Theoverall sodium transport efficiency dropped from 94% to 90% in thistest. The voltage across the membrane peaked at the 200 hours mark sincewe allowed the test to run at lower sodium concentration in the anolyte(<0.5 moles). It is typical in such a situation for the total cellresistance to increase dominated by the electrolyte-membrane interface.

Example 6

A two-compartment electrolytic cell as described in FIG. 1, based on analkali cation-conductive ceramic material membrane 14 was assembled. Thealkali cation-conductive ceramic material 14 is a NaSICON composition(coded: NAS-G). Platinum and nickel were used as anode and cathode.

The electrolytic cell was operated using 5 M aqueous NaOH anolyte and1.5 M aqueous NaOH catholyte. The purpose of this test was to validatethe influence of various cations and anions present in the anolyte onthe electrochemical performance of the ceramic membrane. This test wasoperated at 50° C., and constant current density of 400 mA/cm². Theresults are shown in FIG. 7. High sodium current efficiency (92%) wasobserved again which remained constant during the 350 hours of testing.The voltage drop across the membrane voltage was maintained at a steady4.75 volts.

Example 7

A two-compartment electrolytic cell as described in FIG. 1, based on analkali cation-conductive ceramic material membrane 14 to synthesizesodium hydroxide from transition-metal based sodium salts containingby-product industrial stream. The alkali cation-conductive ceramicmaterial 14 is NaSICON composition (coded: NAS-F). Platinum and nickelwere used as anode and cathode. The electrolytic cell was operated usingthe anolyte Simulant Chemistry 2 and 1.5 M NaOH catholyte. Thecomposition of this anolyte feed was prepared to simulate theradioactively contaminated salt waste stream at Hanford (PNNL), a DOEfacility. Simulant Chemistry 2 is shown in Table 2.

TABLE 2 Anolyte Simulant Chemistry 2 wt % H₂O 68.0460% NaOH 8.5777%Sr(NO₃)₂ 0.0021% NaCl 0.1056% NaF 0.0513% Na₂HPO₄ 0.0989% Na₂C₂O₄0.1569% Na₂CO₃ 1.3832% Na₂SO₄ 1.6212% NaNO₂ 3.3748% NaNO₃ 6.9556% KNO₃0.1237% Ca(NO₃)₂•4H₂O 0.0004% CsNO₃ 0.0016% Ba(NO₃)₂ 0.0021% SiO₂0.0186% AI(NO₃)₃•7H₂O 9.4793%

This test was operated at 50° C. and constant current density of 100mA/cm² for a period of 950 hours. Based on the results shown in FIG. 8,the energy estimates based on the test results is 1900 kWh/ton of NaOHproduction by the ceramic material membrane based electrolytic cell.

Example 8

A two-compartment electrolytic cell as described in FIG. 1, based on analkali cation-conductive ceramic membrane 14. The alkalication-conductive ceramic membrane material 14 is Na₃Zr₂Si₂PO₁₂composition (coded: NAS-D10). Platinum and nickel were used as anode andcathode. The electrolytic cell was operated in batch mode inradioactively contaminated waste. The cell was operated at 25 mA/cm² atambient temperature. FIG. 9 compares the test results of a ceramicalkali ion conducting membrane (coded NASD) with a polymer Nafionmembrane based cell. The results presented in FIG. 9 shows the ceramicNASD membrane transfers almost zero amount of radionuclide, such as Csor Sr, through the membrane. Only sodium ions were selectivelytransported across the ceramic alkali ion conducting membrane from theanode compartment (20) into the cathode compartment (18), while theNafion polymer membrane cell transfers about 60% of the radioactivespecies present in the anolyte feed (26) from the anode compartment (20)into the cathode compartment (18) across the Nafion membrane.

Hydrogen produced in the catholyte compartment 18 may be utilized by afuel cell to generate additional power, the hydrogen can be vented orflared.

The two-compartment electrolytic cell shown in FIG. 1 may be configuredin multi-compartment embodiments. FIG. 10 shows a schematicrepresentation of an electrolytic cell 50 comprising two,two-compartment electrolytic cells arranged in series, separated by abipolar electrode 52. Each two compartment cell in the electrolytic cell50 includes an alkali cation-conductive ceramic material membrane 54used to split an aqueous solution of sodium salts. The alkalication-conductive ceramic material 54 may be NaSICON-type material. Thealkali cation-conductive ceramic material 54 separates a catholytecompartment 58 from an anolyte compartment 60. The sodium ions transportacross the alkali cation-conductive ceramic membranes 54 from theanolyte compartment 60 to the catholyte compartment 58. The gaseousproduct is oxygen in the anolyte compartment 60, and hydrogen in thecatholyte compartment 58. The anolyte compartment 60 is configured withan anode 62 and the catholyte compartment 58 is configured with acathode 64. The bipolar electrode 52 includes the dual properties of ananode 62 a and a cathode 64 a. An electric potential or voltage source65 is provided to operate the electrolytic cell 50.

The electrolytic cell 50 further comprises an anolyte inlet 66 forintroducing chemicals into the anolyte compartment 60 and an anolyteoutlet 68 for removing or receiving anolyte solution from the anolytecompartment 60. The cell 50 also includes a catholyte inlet 70 forintroducing chemicals into the catholyte compartment 58 and a catholyteoutlet 72 for removing or receiving catholyte solution from thecatholyte compartment 58.

The parts of the electrolytic cell 50 may be made of any suitablematerial, including metal, glass, plastics, composite, ceramic, othermaterials, or combinations of the foregoing. The material that forms anyportion of the electrolytic cell 50 is preferably not reactive with orsubstantially degraded by the chemicals and conditions that it isexposed to as part of the process.

The bipolar electrode 52 may be fabricated from a variety of materialsknown in the art, including but not limited to, Kovar alloy(approximately 54% iron, 29% nickel, 17% cobalt), nickel, rutheniumoxide coated on titanium substrate (RuO₂/Ti) dimensionally stable anode(DSA), platinum, platinum coated on titanium substrate, stainless steel,HASTEALLOY® nickel based alloy, INCOLOY® Alloy 800 (iron, nickel, andchromium alloy), and carbon steel.

The operation of the electrolytic cell 50 is similar to that of cell 10,discussed above. There are several potential reactions in the anolytecompartment depending upon the composition of the anolyte inlet stream66. The anolyte inlet stream may be an aqueous feed of sodium basedsalts which may include sodium hydroxide and sodium sulfate and otheralkali and transition metal impurities.

The electrolysis reactions may be as follows:

Anolyte compartment: H₂O→2H⁺+2e ⁻ +½O ₂

2OH⁻→2e ⁻+H₂O+½O₂

Catholyte compartment: 2H₂O+2e ⁻→H₂+2OH⁻

The process can be operated between about ambient temperature and about100° C. The catholyte inlet 70 includes water. It may also include anaqueous solution of sodium hydroxide. The catholyte outlet 72 comprisesan aqueous solution of sodium hydroxide, at a higher concentration thanthe catholyte inlet 70.

The electrolytic cell 50 shown in FIG. 10 may be expanded to includeadditional two compartment cells separated by bipolar electrodes. FIG.11 shows a schematic representation of an electrolytic cell 80comprising four electrolytic cells stacked in series, separated by threebipolar membranes 82.

Each two compartment cell in the electrolytic cell 80 includes an alkalication-conductive ceramic material membrane 84 used to split an aqueoussolution of sodium salts. The alkali cation-conductive ceramic material84 may be NaSICON-type material, as disclosed herein. The alkalication-conductive ceramic material 84 separates a catholyte compartment88 from an anolyte compartment 90. The anolyte compartment 90 isconfigured with an anode 92 and the catholyte compartment 88 isconfigured with a cathode 94. The bipolar electrode 82 includes the dualproperties of an anode 92 a and a cathode 94 a. An electric potential orvoltage source 95 is provided to operate the electrolytic cell 80.

The electrolytic cell 80 further comprises an anolyte inlet 96 forintroducing chemicals into the anolyte compartment 90 and an anolyteoutlet 98 for removing or receiving anolyte solution from the anolytecompartment 90. The cell 80 also includes a catholyte inlet 100 forintroducing chemicals into the catholyte compartment 88 and a catholyteoutlet 102 for removing or receiving catholyte solution from thecatholyte compartment 88.

FIG. 12 shows a schematic representation of electrolytic cell 80′, whichis based upon the general structure of the electrolytic cell 80illustrated in FIG. 11. The electrolytic cell 80′ is designed to producealkali hydroxide, such as sodium hydroxide, in the catholyte solutionproduced in the catholyte compartment. The catholyte solution from onecatholyte compartment is introduced into a second catholyte compartmentto increase the concentration of the alkali hydroxide within the secondcatholyte compartment. As shown, the catholyte outlet 102 from catholytecompartment 88 becomes the catholyte inlet 100′ for the catholytecompartment 88′. This can be repeated in successive catholytecompartments to produce a catholyte outlet solution having increasedalkali hydroxide concentration. FIG. 12 shows one possible configurationof this concept wherein catholyte outlet 102′ becomes the catholyteinlet 100″, catholyte outlet 102″ becomes the catholyte inlet 100″′ forcatholyte compartment 88″′, and catholyte outlet 102″′ contains thefinal catholyte solution with high concentration of alkali hydroxide.

FIG. 13 shows a schematic representation of electrolytic cell 80″, whichis based upon the general structure of the electrolytic cell 80illustrated in FIG. 11. The electrolytic cell 80″ is designed to producealkali hydroxide, such as sodium hydroxide, in the catholyte solutionproduced in the catholyte compartment. The catholyte solution containingalkali metal hydroxide, may be simultaneously received from a pluralityof catholyte compartments in the electrolytic cell. As shown in FIG. 13,the catholyte outlet 102 from several catholyte compartments 88 iscollected into a single catholyte outlet stream 110.

FIG. 14 shows a schematic representation of a multi-compartmentelectrolytic cell 120 comprising four two-compartment electrolytic cellsstacked in a parallel configuration. Each two compartment cell in theelectrolytic cell 120 includes an alkali cation-conductive ceramicmaterial membrane 124 used to split an aqueous solution of sodium salts.The alkali cation-conductive ceramic material 124 may be NaSICON-typematerial, as disclosed herein. The alkali cation-conductive ceramicmaterial 124 separates a catholyte compartment 128 from an anolytecompartment 130. The anolyte compartment 130 is configured with an anode132 and the catholyte compartment 128 is configured with a cathode 134.An electric potential or voltage source 135 is provided to operate theelectrolytic cell 120. An anode lead wire 136 electrically connects thevoltage source to each anode 132. A cathode lead wire 137 electricallyconnects the voltage source to each cathode 134.

The electrolytic cell 120 further comprises an anolyte inlet 138 forintroducing chemicals into the anolyte compartment 130 and an anolyteoutlet 139 for removing or receiving anolyte solution from the anolytecompartment 130. The cell 120 also includes a catholyte inlet 140 forintroducing chemicals into the catholyte compartment 128 and a catholyteoutlet 142 for removing or receiving catholyte solution from thecatholyte compartment 128. The electrolytic cell 120 operates in amanner similar to the other electrolytic cell embodiments disclosedherein. While not shown in FIG. 14, it will be appreciated that thecatholyte outlet 102 from several catholyte compartments 88 may becollected into a single catholyte outlet stream as shown in FIG. 13.

Benefits and applications from two compartment electrolytic cell, eitheralone or combined in a multi-compartment embodiment include, but are notlimited to:

-   -   1. The two compartment electrolytic cell provides the        opportunity to separate and recycle complex industrial alkali        (sodium) based salt feed containing impurities of other alkali,        transitional and salts form main groups to make up to 50%        concentrated sodium hydroxide in the catholyte in the cell        configuration.    -   2. Alkali (or sodium elements) ions can be removed up to ppm        levels from any incoming industrial aqueous byproduct feed        chemistries which makes the process most attractive, efficient,        and economical for industrial salt splitting, separation and        recycling applications.    -   3. An energy efficient approach for electro-synthesis of value        added chemicals from alkali salts with and without the presence        of impurities to make for example, pure sodium hydroxide stream        and reduce or consolidate the volume of the feed anolyte.

While specific embodiments of the present invention have beenillustrated and described, numerous modifications come to mind withoutsignificantly departing from the spirit of the invention, and the scopeof protection is only limited by the scope of the accompanying claims.

1. A method for producing an alkali metal hydroxide, comprising:providing an electrolytic cell comprising at least one membranecomprising ceramic material configured to selectively transport thealkali metal ions, the membrane positioned between an anolytecompartment configured with an anode and a catholyte compartmentconfigured with a cathode; introducing a first solution comprising analkali metal hydroxide solution into the catholyte compartment of theelectrolytic cell such that said first solution is in communication withthe membrane and the cathode; introducing a second solution comprisingat least one alkali metal salt and one or more monovalent, divalent, ormultivalent metal salts into the anolyte compartment of the electrolyticcell such that said second solution is in communication with themembrane and the anode; and applying an electric potential to theelectrolytic cell such that alkali metal ions pass through the membraneand are available to undertake a chemical reaction with hydroxyl ions inthe catholyte compartment to form alkali metal hydroxide.
 2. The methodof claim 1, wherein introducing a first solution into the catholytecompartment and introducing a second solution into the anolytecompartment comprise a continuous operation.
 3. The method of claim 1,wherein introducing a first solution into the catholyte compartment andintroducing a second solution into the anolyte compartment comprise abatch operation.
 4. The method of claim 1, wherein the alkali metalcomprises sodium.
 5. The method of claim 4, wherein introducing a firstsolution into the catholyte compartment comprises introducing sodiumhydroxide as an aqueous solution wherein the concentration of sodiumhydroxide is between about 1% by weight and about 50% by weight of thesolution.
 6. The method of claim 5, further comprising maintaining theconcentration of sodium hydroxide in the catholyte compartment betweenabout 10% and about 20% by weight.
 7. The method of claim 4, furthercomprising maintaining the concentration of the sodium salt in theanolyte compartment between about 1% and about 50% by weight of thesecond solution.
 8. The method of claim 7, further comprisingmaintaining the concentration of sodium in the anolyte compartmentbetween about 5% and about 20% by weight.
 9. The method of claim 4,wherein the ceramic membrane comprises a NaSICON material.
 10. Themethod of claim 4, wherein the ceramic membrane comprises a NaSICONmaterial having the formula Na_(1+x)Zr₂Si_(x)P_(3−x)O₁₂ where 0≦x≦3. 11.The method of claim 4, wherein the ceramic membrane comprises a NaSICONmaterial having the formula, M_(1+x)M^(I) ₂Si_(x)P_(3−x)O₁₂ where 0≦x≦3,where M is selected from the group consisting of Li, Cs, Na, K, or Ag,or mixture thereof, and where M^(I) is selected from the groupconsisting of Zr, Ge, Y, Ti, Sn, Y or Hf, or mixtures thereof.
 12. Themethod of claim 4, wherein the ceramic membrane comprises a NaSICONmaterial having the formula Na₅RESi₄O₁₂ where RE is Y, Nd, Dy, or Sm, orany mixture thereof.
 13. The method of claim 4, wherein the ceramicmembrane comprises a non-stoichiometric sodium-deficient NaSICONmaterial having the formula (Na₅RESi₄O₁₂)_(1−δ)(RE₂O₃.2SiO₂)_(δ), whereRE is Nd, Dy, or Sm, or any mixture thereof and where δ is the measureof deviation from stoichiometry.
 14. The method of claim 4, wherein thesecond solution introduced into the anolyte compartment comprises asodium salt selected from the group consisting of: sodium hydroxide,sodium chloride, sodium carbonate, sodium bicarbonate, sodium sulfate,sodium chlorate, sodium phosphate, sodium perchlorate, sodium nitrite,sodium fluoride, sodium oxalate, sodium organic salts and anycombination thereof.
 15. The method of claim 1, wherein the secondsolution comprises one or more monovalent, divalent, or multivalentmetal salts selected from Na, K, Cs, Ca, Sr, Ba, Al, and mixturesthereof.
 16. The method of claim 1, wherein the second solutioncomprises one or more non-alkali, radioactive metal salts and whereinthe alkali metal hydroxide formed in the catholyte compartment issubstantially non-radioactive.
 17. The method of claim 1, wherein themembrane operates at a current density of between about 20 mA/cm² andabout 200 mA/cm².
 18. The method of claim 1, wherein the sodium-ionconducting ceramic membrane operates at a current density greater than100 mA/cm².
 19. The method of claim 1, wherein the electrolytic cellcomprises a plurality of membranes, each configured to selectivelytransport sodium ions, and at least one bipolar electrode positionedbetween a pair of said membranes such that the electrolytic cellcomprises a plurality of anolyte compartments and a plurality ofcatholyte compartments.
 20. The method of claim 19, wherein alkali metalhydroxide solution is simultaneously received from the plurality ofcatholyte compartments.
 21. The method of claim 20, wherein sodiumhydroxide is received from a first catholyte compartment and introducedinto a second catholyte compartment to increase the concentration of thesodium hydroxide in a sodium hydroxide solution in successive catholytecompartments.
 22. The method of claim 1, wherein the ceramic membranecomprises a material having the formula Na_(1+z)L_(z)Zr_(2−z)P₃O₁₂ where0≦z≦2.0, and where L is selected from the group consisting of Cr, Yb,Er, Dy, Sc, Fe, In, or Y, or mixtures thereof;
 23. The method of claim1, wherein the ceramic membrane comprises a material having the formulaM^(II) ₅RESi₄O₁₂, where M^(II) may be Li, Na, K or Ag, or mixturesthereof, and where RE is Y or any rare earth element.